This invention relates to a circuit for optically controlling input and output currents or output voltage of the circuit and to a semiconductor device for realizing the same.
The optical control circuit of the kind referred to generally comprises a second transistor having a control electrode and a pair of output electrodes forming respectively a collector or drain and an emitter or source and receiving a photo-voltage generated by an array of photovoltaic elements, at the control electrode of the second transistor being controlled so that the input and output currents or output voltage at the output electrodes can be effectively controlled, and eventually a quick operation of the second transistor can be effectively assured by an additional circuit including a first transistor.
While various types of the optical control circuit of the kind referred to have been suggested, an optical control circuit for turning on and off, for example, a field effect transistor (which shall be simply referred to as "FET" hereinafter) has been disclosed in U.S. Pat. No. 4,500,801 to William J. Janutka, in which a diode is connected as a constant voltage element between a control electrode and a source of a first FET, a resistor connectable in parallel with photovoltaic elements is connected between the control electrode and a drain of the first FET, and a second transistor is connected at its control electrode to the source of the first FET. In this case, a voltage applied to the control electrode of the second FET with respect to the source of the second FET to drive the first FET into a conducting state, and the second FET is to be thereby turned off.
In this patent to Janutka, the first FET has been difficult to be formed in the same substrate as that of the second FET without giving any influence on inherent characteristics of the second FET or adding complicated fabrication process steps, and there has been left a problem unsolved in that the known circuit has been difficult to be prepared in an integrated manner, so as to remain high in manufacturing costs and poor in the productivity.